using System;
using System.Linq;

class Program
{
    static void Main()
    {
        var a = Console.ReadLine().Split().Select(double.Parse).ToArray();
        var b = Console.ReadLine().Split().Select(double.Parse).ToArray();

        a[0] = -a[0];

        double ans = a[1] + (b[1] - a[1]) / (b[0] - a[0]) * (-a[0]);

        Console.WriteLine(ans);
    }
}